The invention relates to a tank system for a motor vehicle comprising a volume modifying element, which is provided in the inner chamber of the fuel tank and which is used to equalize the vapor pressure. The so-called “compensation volume” of said volume modifying element is connected to the surrounding area. With respect to the prior art, in addition to DE 10 2008 046 586 A1, reference is made in particular to U.S. Pat. No. 5,460,135. Instead of the term “volume modifying element”, the term “volume modifying expansion means” or—at least to the extent that it relates to the present invention—the term “vapor pressure equalizing device” can be used as well.
Nowadays, the legal requirements imposed for zero-emission motor vehicle tank systems are extremely high. Under nearly any conceivable circumstances, taking into account the various country-specific regulations (the various countries worldwide issue different regulations), practically no gaseous fuel components (generally, these are hydrocarbons) are supposed to escape from the inner chamber of the tank into the surrounding area. It is known that such gaseous fuel components accumulate in particular in large quantities when filling the fuel tank with new fuel, but also after a prolonged standstill of the motor vehicle as a result of an increase in temperature. In connection with the latter, one skilled in the art is familiar with the term of “diurnal losses”, which is to say, the loss(es) of gaseous hydrocarbon emissions that have to be removed from the inner chamber of the tank after a prolonged standstill of the motor vehicle due to temperature fluctuations (for example, resulting from the change from day to night) to avoid creating an impermissibly high overpressure inside the tank. However, hydrocarbon emissions that form in the tank due to temperature changes in the operation of the motor vehicle also have to be removed, wherein maintaining the respective vapor pressure equilibrium in the tank always leads to such emissions.
It is known that at least those emissions that do not accumulate when the fuel tank is being filled are temporarily stored in an activated carbon filter or the like, which in the present case is generally referred to as a storage unit for gaseous fuel components, until a combustion engine supplied with fuel from the fuel tank (and generally provided as vehicle drive aggregate) is suitably operated so that said storage unit can then be flushed. The fuel components stored temporarily therein are led to the combustion engine for combustion. For the sake of completeness, it should also be mentioned that depending on the country, it is either common practice to suction off the hydrocarbon emissions that accumulate during the refueling of the tank either during the refueling at the service station (as, for example, in the European Union=“ECE system”), or to store them also in an activated carbon filter, which then has to have appropriately large dimensions, or in an appropriate storage unit (such as required in the USA as ORVR=Onboard Refueling Vapor Recovery, for example).
In view of avoiding the aforementioned “diurnal losses” or other operations-related hydrocarbon emissions, pressure tanks have already been proposed, or are in serial production, in which a correspondingly high overpressure (as well as a small negative pressure) can prevail so that, aside from extreme exceptional cases, no fuel vapors have to be removed from the tank. However, such pressure tanks are quite complex.
Furthermore, measures are known to reduce the formation of gaseous fuel components in the inner chamber of the tank, in particular in connection with a refilling process of the fuel tank; see the aforementioned U.S. Pat. No. 5,460,135. This is because it applies for the United States that all gaseous hydrocarbon emissions accumulated during the filling of the fuel tank of a vehicle must be temporarily stored in a storage unit for gaseous fuel components in the vehicle. To reduce the quantity of said emissions, the aforementioned reference provides for a flexible air bag in the fuel tank, which has its minimum volume when the tank is completely filled with fuel and, as liquid fuel is removed from the tank, is continuously filled with ambient air, whereas an emptying of said air bag is prevented. In this way, by nature, only a smaller quantity of fuel vapors can form above the liquid level in the tank, as would otherwise be the case in the same tank without such an air bag. In connection with a new refilling of the tank, the air bag is then emptied through the storage container or activated carbon filter into the surroundings.
DE 10 2008 046 586 A1 (also mentioned above) also describes a flexible air bag or the like in an inner chamber of a tank, the outer skin of which is in fluid connection with the fuel in the inner chamber of the tank (which is to say, it protrudes into it). The inner chamber of the air bag is in connection with the atmosphere, which is to say the ambient air. The air bag, which is described as expansion means in the reference, can be designed in such a fashion that with the volume changes of the fluid in the tank, which are possible in the entire operating temperature range of the tank designed as a closed system, no or nearly no increase in the fluid level (=level of liquid) occurs in the tank, which is to say, that the entire or almost the entire volume change of the fluid in the tank is compensated by the expansion means. This is to reliably prevent the escape of harmful gases into the atmosphere and facilitate an expansion of the fuel occurring during an increase in temperature.
The present invention then builds on the finding that because of the vapor pressure equilibrium in a fuel tank that is filled almost completely with liquid fuel, less gaseous fuel components are created above the liquid level than in the same fuel tank if said fuel tank is filled only half, for example, or even less. Consequently, with a flexible air bag or the like, which is provided above the liquid level created by liquid fuel in the inner chamber of the tank, the quantity of fuel components that transition from the inner chamber of the tank into the gaseous state can be reduced. Furthermore, instead of talking about an “air bag”, this is then generally referred to as a volume modifying element that has or includes a modifiable compensation volume that is connected to or can be connected to the surroundings. In this way, the volume modifying element can practically breathe and react to changed boundary conditions.
There is therefore needed a tank system of a motor vehicle having a volume modifying element provided in the inner chamber of the fuel tank, the so-called compensation volume of which is in connection with the surroundings depending on certain boundary conditions, with which the current statutory regulations of important countries worldwide, in particular also the United States of America, can be further improved upon with respect to evaporations emissions with vehicle tanks with a justifiable or reduced effort, or maintained with less effort.
This need is met by a tank system of a motor vehicle with a volume modifying element provided in the inner chamber of the fuel tank, the so-called compensation volume of which is connected to the surroundings depending on various boundary conditions, having a combination of the following characteristics.
The only connection between the compensation volume of the volume modifying element and the surroundings is guided through a storage unit for gaseous fuel components, and the inner chamber of the fuel tank is connected to the surroundings via a selectable valve unit, which is generally open when the fuel tank is being filled as well as when an overpressure limit value (between the pressure in the inner chamber of the tank and the surrounding pressure) is exceeded in a magnitude of 5 mbar to 100 mbar, and when it falls below a specific negative pressure limit value (between the pressure in the inner chamber of the tank and the surrounding pressure), the amount of which is not larger than the overpressure limit value, and is otherwise closed, as well as a storage unit for gaseous fuel components connecting to said valve unit.
According to the invention, a volume-variable expansion elements or the like, which is connected to the surroundings (or the atmosphere)—referred to here as a volume modifying element—is provided in the inner chamber of the tank, and, for example, principally known from the aforementioned prior art by the term “flexible air bag” or “expansion means”. According to the invention, the so-called compensation volume of the volume modifying element, which is to say the capacity of the volume modifying element, is connected to the surroundings or atmosphere exclusively via a storage unit for gaseous fuel components. As a result, ambient air, which, because of a pressure in the inner chamber of the tank that is at least slightly below the air pressure in the surroundings of the vehicle equipped with the tank system according to the invention, flows from the surroundings into the volume modifying element and always has to pass through the respective storage unit before it reaches the compensation volume. Likewise, air that is removed from the compensation volume of the volume modifying element into the surroundings because of a pressure in the inner chamber of the tank, which is higher than the entire overpressure limit value above the air pressure in the surroundings of the vehicle equipped with the tank system according to the invention, must and can always pass through said storage unit until it reaches the surroundings. This ensures (with respect to the outflow of air from the volume modifying element into the surroundings) that gaseous fuel components that reach or diffuse from the inner chamber of the tank, through the somewhat flexible wall of the volume modifying element for the representation of the volume modification, into said compensation volume are not removed into the surroundings because gaseous fuel components from the compensation volume are held back in the storage unit when the air containing such gaseous fuel components passes the storage unit. On the other hand, when air flows from the surroundings into the volume modifying element through the storage unit, the storage unit is slightly flushed so that the air introduced into the volume modifying element already contains gaseous fuel components. This can reduce—albeit slightly, but nevertheless—the tendency that through its wall additional gaseous fuel components diffuse from the tank into the inner chamber of the volume modifying element. Furthermore, an inflow of air from the surroundings into the volume modifying element is also required to compensate for the increase of the volume above said liquid level, which occurs when liquid fuel is removed from the tank, so that the air introduced to compensate the volume of removed fuel later also only has to be removed in a lesser portion.
If applicable, the volume modifying element or its compensation volume can practically act as an additional storage unit for gaseous fuel components, e.g. in that air flowing from the surroundings through the actual storage unit for gaseous fuel components into the volume modifying element carries along a part of the gaseous fuel components stored in the storage unit, which are then stored in the compensation volume of the volume modifying element. Although such gaseous fuel components have to be stored in the interim again in said storage unit when part of the air possibly flows from the volume modifying element back into the surroundings at a later time, this is made possible by the system and advantageously the storage unit, which is to say its usual activated carbon exhaust or generally its sorption means, are held closer in the range of its equilibrium and can therefore be better regenerated when fresh ambient air flows through it. In particular, however, the volume modifying element according to the invention allows a fluctuation of the partial pressure in the tank above the tank liquid level within certain limits, with the overall pressure in the fuel tank essentially remaining unchanged.
The aforementioned possible exchange of air between the compensation volume of the volume modifying element and the surroundings is or can occur in a vehicle that has been at a standstill for an extended period of time (such as longer than 24 hours, for example). This is, in particular, because of the ensuing temperature changes in the tank, which, as is known, is replicated in various statutory emissions tests (such as for the temperature range of 18° C. to 42° C.). As regards the facultative interim storage of gaseous fuel components in the volume modifying element, as mentioned above, and the partial flushing of the aforementioned storage unit connected thereto, it should be mentioned, in order to avoid misunderstandings, that the actual regeneration of said storage unit, which is to say, a practically final flushing of the same, occurs only after air from the surroundings is supplied through said storage unit to the aforementioned combustion engine or the like, for which the tank system according to the invention is provided to supply fuel, for combustion.
Furthermore, according to the invention, the tank system is developed as a practically closed negative pressure storage system, which is to say, the fuel tank and the tank system are designed such that there can be a slight pressure difference in a magnitude of up to ±100 mbar (millibar) between the pressure in the inner chamber of the fuel tank (and thus outside the volume modifying element) and the surroundings. In other words, in the inner chamber of the tank there can be up to (in the magnitude of) 100 mbar overpressure or negative pressure; however, a limit value in a smaller amount may also be provided for negative pressure. It goes without saying that depending on the design and, in particular, depending on the maximum possible size of the compensation volume of the volume modifying element in relation to the volume of the fuel tank and/or of the (also with respect to the materials being used) selected development of the volume modifying element, a lesser overpressure limit value than 100 mbar may be specified, for example also a relatively low overpressure limit value of 10 mbar. So as to ensure that these limit values for overpressure and negative pressure are maintained, a valve unit suitable for this purpose is provided in a so-called tank ventilation line, through which the inner chamber of the tank is finally connected to the surroundings and in which, of course (as is principally known) a storage unit for gaseous fuel components is provided.
The aforementioned valve unit can ensure, for example, that there is a desired connection between the inner chamber of the tank and the surroundings when the fuel tank is filled with fresh fuel. That is because the latter is needed when the tank is filled (=refueling) to either remove fuel vapors from the tank via an activated carbon filter or to suction small amounts of ambient air through the connection if the suctioning off of the fuel vapors from the tank by the gas station occurs via the gas filling pipe and the gas suction nozzle inserted therein. The valve unit can open, for example, even if one of the referenced pressure differential limit values are exceeded when the temperature in the fuel tank rises to higher values during operation of the vehicle than would normally be the case with a parked motor vehicle, or as is reproduced in the aforementioned statutory emissions tests.
It was found that with a volume modifying element according to the invention, it is possible to pass the current statutory emissions tests for motor vehicle tank systems while adhering to the aforementioned pressure differential limit values (in the magnitude of up to approx. ±100 mbar) in the inner chamber of the tank without any problems. With the exception of refueling, which is to say filling the fuel tank, the valve unit practically does not have to open in the tank ventilation line when performing the diurnal emissions tests when the vehicle is parked (which is to say, not operated), which is to say usually (except for very few exceptions) if the maximum possible capacity of the compensation volume of the volume modifying element is in a magnitude of preferably 25% to 40% of the inner chamber of the tank. It is not necessary that the volume modifying element rests with one of its walls fully on the liquid surface of the liquid fuel in the tank; rather, the volume modifying element can be, for example, as shown in the DE 10 2008 046 586 A1 mentioned earlier, developed as an air bag that extends only over a small partial area of the inner chamber of the tank, wherein it is also specifically not necessary that—as described in the aforementioned reference—the wall of the volume modifying element is in fluid connection with the fuel in the tank.
It has already been mentioned that the volume modifying element of a tank system according to the invention can be developed in the form of an elastic or in another way deformable bubble or such a bag (for example, present in folded form in a state of reduced capacity), But it can definitely also be designed differently. What is essential is that the volume modifying element has a suitably deformable wall, with the help of which the capacity or the compensation volume of the volume modifying element can be modified in a broad scope, which is to say from nearly zero up to (as already facultatively stated) preferably 25% to 40% of the inner chamber of the tank. In doing so, it is always possible, even in case of an elastic bubble or an elastically deformable bag, that a deformable wall of the volume modifying element completely encases the compensation volume. Alternatively, however, the aforementioned modifiable compensation volume, which is connected to the surroundings, can also be formed by a suitable deformable wall element of the volume modifying element in cooperation with a section of the inner wall of the fuel tank.
According to an advantageous refinement, a protective splatter wall may be provided in the fuel tank. This wall encloses the volume modifying element in its greatest possible expansion at least in sections and has through-openings for fuel. To limit the mechanical load on the deformable volume modifying element caused by sloshing movements of the fuel in the fuel tank during driving of the motor vehicle, walls (or at least one such wall) provided with holes or the like are proposed, which practically form a protective box around the expanded volume modifying element. The holes in the wall or in the slosh protection walls are developed such that when filling the fuel tank, which is to say when refueling, the difference between the liquid level inside and outside of the “protective box” is no more than two centimeters in height. This partially permeable slosh-protective wall thus forms a flow resistance for sloshing movements of the fuel in the tank and reduces the pressure load on the volume modifying element resulting therefrom.
In particular at a high filling level, which is to say, a high liquid level in the fuel tank, the volume modifying element may, if applicable, partially dip into the fuel. Thus, if the fuel level is measured according to the principle of a lever sensor, which has been widely used to date, the electronic evaluation unit of such a filling level measuring device should also in addition take into account, if applicable, a negative pressure in the fuel tank (by means of a pressure sensor) to prevent measuring errors due to the aforementioned immersion. This makes it possible to draw, by calculation, conclusions as to the immersion depth of the volume modifying element in the liquid level, for example, even in the case of an unfolding bag (as volume modifying element), if the folding behavior of said bag is known. Taking negative pressure in the tank into account in the evaluation of a lever sensor signal, as is proposed here, is also possible using an experimentally (for various liquid levels) determined characteristic diagram, or there is the option that in case of negative pressure in the tank, the current measuring signal of the lever sensor is not taken into account at all, but rather recent measuring values that are corrected with current fuel usage values, if applicable, are relied upon.
A volume modifying element that works according to the invention should be permeation-proof, if possible, and capable of performing many thousands of volume enlargement processes and volume reduction processes. To that end, the volume modifying element can be developed in form of a foldable bubble or a purposely foldable bag, which is to say, it should be designed to unfold automatically, for example like an Origami technique, for a volume enlargement, and fold up automatically for a volume reduction. This also allows the use of a sufficiently dense material (also dense with respect to permeation). Here, reference is made to such foldable structures, for example Origami-like structures or containers, for example, described in the article “Inflatable Cylinders for Deployable Space Structures” by Mark Schenk et al, which can be found in the Internet under http://www.markschenk.com/research/files/schnek2013-Transformables.pdf, or also to the article “Review of Inflatable Booms for Deployable Space Structures: Packing and Rigidization” under http://www-g.eng.cam.ac.uk/advancedstructures/files/pdf/2014JSR.pdf.
The storage unit for gaseous fuel components provided in the connecting line between the compensation volume of the volume modifying element and the surroundings can be developed so as to take up essentially gaseous fuel components, which diffuse through the wall of the volume modifying element into its compensation volume (AV) and be present there in extremely low concentration in the magnitude of several thousand ppm (parts per million). However, activated carbon filters generally provided with fill are not able to absorb gaseous hydrocarbons from such small concentrations, which is why a smaller storage unit is proposed here for gaseous fuel components, which is described as “low bleed emission activated carbon filter” or “honeycomb” and is known in this form in particular from open vehicle tank systems intended for the United States.
However, alternatively, the aforementioned storage unit can also be designed to take up, additionally or primarily, such gaseous fuel components as are removed in the operation of the motor vehicle because of too high temperatures in the tank, or in the filling of the fuel tank with fresh fuel via the aforementioned, then open valve unit from the fuel tank. In that case—as already mentioned earlier—the compensation volume of the volume modifying element can function as additional storage for gaseous fuel components, namely in that air flowing from the surroundings through the actual storage unit for gaseous fuel components into the volume modifying element carries along a portion of the gaseous fuel components stored in the storage unit, which are then stored in the compensation volume of the volume modifying element.
According to another alternative, the tank system according to the invention can, in its entirety, comprise two storage units for gaseous fuel components, namely a first larger storage unit which, viewed from the inner chamber of the tank, connects to the aforementioned valve unit (also referred to as negative pressure tank locking valve), which acts as an activated carbon filter for the refueling and/or for high tank pressure values in the operation of the motor vehicle, as well as a second smaller storage unit such as the aforementioned “low bleed emissions activated carbon filter” or “honeycomb”, which in the tank ventilation line, when viewed from the inner chamber of the tank in the direction of the surroundings, connects to the first storage unit. A preferably single connecting line with the compensation volume of the volume modifying element can then either extend into a connection between the first storage unit and the second storage unit (for gaseous fuel components), or alternately, when viewed from the volume modifying element, into the first storage unit or upstream of the same into a tank ventilation line comprising the valve unit downstream of the valve unit.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.